1. Field of the Invention
The present invention relates to an optical information recording medium, more particularly to a phase-change type optical information recording medium.
2. Discussion of Background
There is conventionally known a phase-change type optical information recording medium which utilizes phase changes between a crystalline phase and an amorphous phase or between one crystalline phase and another crystalline phase as one of the optical recording media which are capable of recording, reproducing and erasing information, for instance, by the application of a laser beam thereto. This kind of phase-change type optical information recording medium is capable of overwriting information by the application of a single laser beam thereto.
Such a phase-change type optical information recording medium is generally composed of a substrate made of polycarbonate, a first protective layer provided on the substrate, a recording layer provided on the first protective layer, a second protective layer provided on the recording layer, and a light reflection and heat dissipation layer.
Representative examples of recording materials for use in the above-mentioned recording layer are the so-called chalcogen-based alloys, such as Ge-Te, Ge-Se-Sb and Ge-Te-Sn as disclosed in U.S. Pat. No. 3,530,441, and Ge-Te-Se-Sb and Ge-Te-Sb, with the composition ratios of such elements being specifically selected for improvement of the recording/erasing repetition performance thereof as proposed in Japanese Laid-Open Patent Applications 62-73438 and 63-228433.
Furthermore, as the materials for the above-mentioned protective layers, there are conventionally known oxides such as SiO and A1.sub.2 O, and nitrides such as BN, Si.sub.3 N.sub.4 and AlN.
However, protective layers composed of such an oxide or nitride are known to have the shortcoming that the protective layers are thermally damaged by the heat applied thereto during the cycles of heating with the application of a laser beam and cooling, resulting in the reduction of the reliability of the performance of the optical information recording medium comprising such a protective layer.
In order to eliminate the above-mentioned shortcoming, mixed materials, such as a material composed of ZnS and SiO.sub.2, are proposed as disclosed in Japanese Laid-Open Patent Application 63-259855 and Japanese Patent Publication 4-74785. It is reported that by using such a material in the protective layer, the above-mentioned recording/erasing repetition performance can be significantly improved for high line speed recording media, but the recording/erasing repetition performance cannot be sufficiently improved for low line speed recording media such as compact disks.